1. Field of the Invention
The present invention relates to rotating electrical machines mounted on vehicles and, more particularly, the present invention relates to a structure of a stator core adapted to a rotating electrical machine.
2. Description of the Related Art
Conventionally, a type of rotating electrical machine having a stator core has been disclosed. In the rotating electrical machine, the stator core is designed to be divided into a plurality of cores each having an arc shape and the divided cores are secured in a circular form. It is noted that the rotating electrical machine is used in an electric motor and a generator both mounted on an electric vehicle and/or a hybrid car.
For instance, there has been disclosed a stator core of a rotating electrical machine in Japanese Unexamined Patent Publication No. 2009-11063. As shown in FIG. 2, this stator core 2 includes divided cores 1 secured in a circular form. As shown in FIG. 1, each of the divided cores 1 has an arc shape and is made of laminated soft magnetic thin plates. As exemplified in FIG. 3, the stator core 2 having the divided cores 1, which form a circular core group, can be inserted into a tubular steel ring 3 by means of a shrinkage fit to be secured. In this securing method, the ring 3 is heated in advance to widen the aperture thereof thereby to allow the divided cores 1 in a circular form to enter the aperture. After the divided cores 1 enter inside the ring 3, the ring 3 shrinks as it returns to the normal temperature, thereby tightening and securing the divided cores 1 forming the circular form by means of the compressive stress of the ring 3.
In the conventional stator core 2, however, a motor loss is deteriorated caused by the compressive pressure of the ring 3 loading on the divided cores 1, as exemplary shown in FIG. 4. FIG. 4 shows a relationship between the compressive load (N) loading on the divided cores 1 having the circular form, and a loss density (kW/m). When the frequency is 100 Hz (f=100 Hz) and 200 Hz (f=200 Hz), which are equivalent to the excitation frequency of the rotating electrical machine, the compressive load gradually increases, whereby the loss density gradually increases, and therefore the motor loss is deteriorated. When the frequency is 400 Hz (f=400 Hz), the loss density apparently increases in the range of 0 to 10 N in the compressive load.
Moreover, as exemplary shown in FIG. 5, when the compressive pressure is loaded on the divided cores 1 having the circular form, the magnetic permeability is deteriorated, thereby causing the magnetic flux to less likely pass through the stator core 2. FIG. 5 shows a relationship between the compressive load (N) of the compressive pressure loading on the divided cores 1 having the circular form, and the relative magnetic permeability. When the frequency is 100 Hz (f=100 Hz), 200 Hz (f=200 Hz) and 400 Hz (f=400 Hz), which are proportional to the number of revolutions of the rotor, the relative magnetic permeability gradually decreases as the load gradually increases. This means that the magnetic permeability decreases and thereby causing the magnetic flux to less likely passes. In this case, the current value for forming a magnetic field increases, and thereby a copper loss generated in the coil increases.
As described above, when the compressive pressure of the ring 3 is applied to the divided cores 1 having the circular form, the magnetic property of the stator core 2 is deteriorated, and the motor loss is increased.